Topic: Communication Systems
A 10 dB attenuator is put at the input of a low noise amplifier having 3 dB noise figure. Now the noise figure of the cascaded amplifier will be
- A. 3 dB
- B. 12 dB
- C. 7 dB
- D. none of the above
Answer: D. none of the above
Explanation: The noise figure of the attenuator is equal to its attenuation, so $NF_1 = 10\text{ dB}$. The noise figure of the cascaded system is approximated by $NF_{total} \approx NF_1 + NF_2 = 10\text{ dB} + 3\text{ dB} = 13\text{ dB}$. Since 13 dB is not in the options, the correct choice is 'none of the above'.